This invention relates to a novel apparatus for rotating a patient during stereotactic radiosurgery and a novel method for performing stereotactic radiosurgery.
There is at present an increasing interest by a number of radiation therapy groups to carry out stereotactic radiosurgery using radiation from sources such as medical accelerators (x-ray beams) or cobalt 60 units (gamma ray beams). The primary goal of such therapy is to deliver high doses of radiation to small target volumes within the brain and at the same time minimize the radiation dose to all points outside the target volume.
Several techniques have been developed for isocentrically mounted linear accelerators in which the gantry is moved during therapy. One of these, the dynamic rotational method, is carried out by arcing the gantry and moving the treatment table at the same time. Podgorsak et al., "Dynamic Stereotactic Radiosurgery." Int. J. Radiat. Oncol. Biol. Phys. 14:115-125 (1988). A second technique, the multiple arc method, employs several non-coplanar arcs with the treatment table moved between arcs. Lutz et al., "A System for Stereotactic
Radiosurgery with a Linear Accelerator," Int. J. Radiat. Oncol. Biol. Phys. 14:373-381 (1988); Colombo et al., "External Stereotactic Irradiation by Linear Accelerator," Neurosurgery 16:154-160 (1985); Hartman et al., "Cerebral Radiation Surgery Using Moving Field Irradiation at a Linear Accelerator Facility," Int. J. Radiat. Oncol. Biol. Phys. 11:1185-1192 (1985).
A common requirement for the conventional multiple arc and dynamic rotation methods is that the axis of rotation of the treatment table intersect the isocenter of the gantry for all positions of the gantry and treatment table used for patient therapy. That requirement adds to the complexity of patient positioning. Additionally, neither technique allows a full 360.degree. rotation of the gantry due to possible gantry collisions with the treatment table, patient, and the stereotactic equipment. A full rotation of the gantry is desirable because it tends to produce isodose contours that are more symmetrical and smooth than those generated by multiple arcs.
Moreover, in these conventional methods, the patient is in a supine position on a treatment table with his head held immobile by a stereotactic head ring which is screwed into the patient's skull. Any unintentional movement of the treatment table may cause injury to the patient because the stereotactic head ring that is screwed into the patient's skull is mounted to the floor and cannot move coincidently with the treatment table.
A further shortcoming of existing rotation methods and associated respective apparatus relates to a tendency for the gantry to move after the drive motor is stopped. Some accelerators have been equipped with collision switches that turn off power to the gantry drive motor in case of a failure of the gantry to stop at the intended angle or in case of a mistake in setting the gantry or table stop angles. Because of the inertia created when the gantry is arced, the gantry tends to move as much as 10.degree. after the drive motor is stopped, thereby creating a condition in which the gantry may collide with the patient or stereotactic equipment.